The process of refrigeration involves moving heat from a chamber or surface to be cooled, and rejecting that heat at a higher temperature than an ambient medium (e.g., air). Vapor compression-based cooling systems have a high coefficient of performance (COP) and are commonly used for cooling chambers and surfaces. Conventional vapor compression-based refrigeration systems utilize a thermostatically regulated duty cycle control. Such systems typically are not dynamic enough to meet both steady state and transient demand (such as during pull down or recovery), and therefor include excess cooling capacities that far exceed heat extraction demand required during steady state operation. Excess cooling capacity allows improved pull down performance, but due to the nature of their control, thermodynamic limits, and product performance demands, conventional vapor compression systems are less efficient than optimum. Excess cooling capacity also entails large current surges during start-up and requires more expensive electrical components.
The sub-optimum efficiencies of vapor compression-based refrigeration systems relate to the desire for such systems to precisely control the temperature within a cooling chamber. Typically, when a temperature within a cooling chamber exceeds a specified value a vapor compression-based refrigeration system is activated and continues to run until the temperature in the cooling chamber is below the specified value—at which point the vapor compression-based system is turned off. This type of control scheme typically has a relatively large control band and a relatively large internal temperature stratification to seek to minimize energy consumption and allow for operation in varied ambient conditions. Such a control scheme is most often utilized because throttling or capacity variation is difficult and expensive to implement with the vapor compression cycle, and throttling or capacity variation provides limited efficacy as volumetric efficiency falls.
Vapor compression based systems also frequently use chlorofluorocarbon (CFC)-based refrigerants; however, the use of CFC-based refrigerants pose an environmental threat since release of such compounds may lead to depletion of the Earth's ozone layer.
Thermoelectric cooling systems represent an environmentally friendly alternative to vapor compression systems, since they do not require CFC-based refrigerants. Thermoelectric coolers (also known as thermoelectric heat pumps) produce a temperature difference across surfaces thereof in response to application of an electric current. Heat may be accepted from a surface or chamber to be cooled, and may be transported (e.g., via a series of transport pipes) to a reject heat sink for dissipation to an ambient medium such as air. Thermoelectric cooling systems may include passive heat reject subsystems. such as thermosiphons or heatpipes, that dispense with a need for forced transport of pressurized coolant though a reject heat sink. As with all refrigeration systems, the smaller the temperature difference across a thermoelectric heat pump, the more efficient the heat pump will be at transporting heat. Despite the environmental benefits of thermoelectric cooling systems, however, such systems have COP values that are typically less than half of vapor compression systems. Enhancing COP of thermoelectric cooling systems and enabling their use over a wide range of ambient temperature conditions would be beneficial to promote increased adoption of such systems.